The invention concerns a method for preparing decorative coatings on metals, especially on aluminum, titanium, niobium, zirconium, tantalum, and their alloys, preferably for the jewelry industry, handicrafts, device construction, claddings and coins, metal-plastic art, and medals.
At the present time special multi-stage procedures are used to obtain nonshiny, matte surfaces on relatively cheap, easily machinable, and processable metals. These procedures roughen, structurize, coat, color, and seal the surface.
On conventional metals of the jewelry and custom jewelry industry, e.g. silver, gold, platinum, such matte surfaces cannot be achieved directly and can be achieved indirectly only through organic or inorganic coatings layers. But visually they reduce the value of the noble and expensive base material. Known methods for producing decorative and colored metal surfaces include the Kalcolor process or color anodizing (U.S. Pat. No. 3,631,384). Here the color is generated directly by the electrolyte, or organic dyes or inorganic color pigments are subsequently placed into the transparent carrier layer in order to provide color. This carrier layer is produced, for example in the case of aluminum, by anodizing with direct or alternating current in sulfuric-, phosphoric-, maleic-, salicyl-oxalic-acid electrolytes. But such surfaces always exhibit a "cold" sheen. Furthermore, these relatively expensive multi-stage processes can achieve a homogeneous surface decoration on e.g. plastic art, medals, hollow bodies, delicate structures, brooches, and the like either not at all or only with great technical effort. Furthermore, it is known that transparent coatings of aluminum oxide, produced by the anodic oxidation of aluminum, can subsequently be colored with various dyes (U.S. Pat. No. 3,031,387). Since these coatings are transparent, the metallic sheen of the surface always remains preserved. The organic compounds used to dye the coatings are furthermore affected by environmental factors such as radiation, temperature, and humidity, and change their hue.
Various decorative metal surfaces are also known in the jewelry industry. For example, in patent No. DD-WP 204845 the utilization of electronic parts as jewelry is described and in patent No. DE-OS 1446289 a special enameling process for decorative coatings on jewelry is presented. Colored, anodized aluminum parts are also used as jewelry elements.